Your damage is calculated instantaneously, right before the finish plays.
Your recast is calculated instantaneously, right before the ready animation plays.
For your bp- set to work, it needs to be on when recast is calculated.
For any damage increasing gear to work, it needs to be on when damage is calculated.
Gear only matters at those 2 precise times, there's nothing you can manipulate to make it better or make more than one piece per slot relevant.
OP is clearly not a native english speaker and the terminology used in spellcast/gearswap is probably confusing them.